Antigen-nonspecific intercellular interactions between T-lymphocytes and antigen-presenting cells (APCs) generate T cell co-stimulatory signals that generate T cell responses to antigen (Jenkins and Johnson (1993) Curr. Opin. Immunol. 5: 361-367). Co-stimulatory signals determine the magnitude of a T cell response to antigen and whether this response activates or inactivates subsequent responses to antigen (Mueller et al. (1989) Annu. Rev. Immunol. 7: 445-480). T cell activation in the absence of co-stimulation results in an aborted or anergic T cell response (Schwartz, R. H. (1992) Cell 71: 1065-1068). One key co-stimulatory signal is provided by interaction of the T cell surface receptor CD28 with B7-related molecules on antigen presenting cells (e.g., B7-1 and B7-2, or CD80 and CD86, respectively) (P. Linsley and J. Ledbetter (1993) Annu. Rev. Immunol. 11: 191-212). The interaction of CD28 with B7-1 (CD80) and B7-2 (CD86) co-stimulatory molecules provides a major signaling pathway for augmenting and sustaining T cell responses (Freedman et al. (1987) J. Immunol. 137: 3260-3267; Freeman et al. (1989) J. Immunol. 143: 2714-2722; Freeman et al. (1991) J. Exp. Med. 174: 625-631; Freeman et al. (1993) Science 262: 909-911; Azuma et al. (1993) Nature 366: 76-79; Freeman et al. (1993) J. Exp. Med. 178: 2185-2192).
CD28 is constitutively expressed on the surface of T cells, virtually all human CD4+ T cells, to a lesser extent on human CD8+T cells, some natural killer cells and all murine T cells. CD28 is a type I transmembrane glycoprotein and is a member of the Immunoglobulin family by virtue of its single Ig variable-like extracellular domain which has a MYPPPY (SEQ ID NO: 639) motif required for binding CD80 and CD86 (Peach et al. 1994, J. Exp. Med. 180: 2049-2058). CD28 has a cysteine residue located after the Ig variable-like domain, which is involved in its homodimerization. The protein sequence of CD28 and a nucleic acid encoding a human CD28 are disclosed, for example, in Harper et al. J. Immunol. (1991) 147: 1037-44. The sequence of a human mRNA encoding CD28 also is disclosed in NCBI Accession No. NM_006139, last updated Apr. 19, 2009, for example. The complete protein sequence of a human CD28 also is disclosed in NCBI Accession No. NP_006130, last updated Apr. 19, 2009, for example.
CD28 transmits a signal that synergizes with the T cell receptor (TCR) signal to promote the activation of naïve T cells (Lanzavecchia et al. (1999) Cell 96: 1-4). CD28 signaling regulates the threshold for T cell activation and significantly reduces the number of TCR engagements needed for effective T cell activation (Viola et al. (1996) Science 273: 104-6). CD28 co-stimulation results in enhanced T cell proliferation, production of multiple cytokines and cytokine receptors, increased expression of proteins involved in cell cycle progression, sustaining T cell survival, and sustained CD40Ligand (CD40L) expression on T cells (Sharpe et al. Fundamental Immunology, W. E. Paul Ed. Fifth Edition, Page 396).
CD28 signals have a critical role in regulating CD4 and CD8 T cell differentiation. CD28 also optimizes the responses of previously activated T cells, promoting IL-2 production and T cell survival. IL-4 production by naïve T cells is highly dependent on B7-1/B7-2 co-stimulation. Interruption of the CD28/B7 pathway during activation of naïve T cells impairs T cell proliferation and differentiation, while interruption of the CD28/B7 pathway in previously activated T cells diminishes T cell expansion but not effector cytokine production (Sharpe et al. Fundamental Immunology, W. E. Paul Ed. Fifth Edition, pages 393-404).
T helper cell-dependent antibody responses use the B7-CD28 pathway to provide co-stimulatory signals essential for cognate T cell/B cell interactions required for Immunoglobulin class switching and germinal center formation. In CD28 knock-out mice, potentially reactive B cells accumulate within lymphoid follicles after antigenic stimulation, but are not able to proliferate or undergo somatic mutation, (Ferguson et al. (1996) J. Immunol. 156: 4576-4581).
B7-1 and B7-2 are also ligands for a second, higher affinity receptor, CTLA4 (CD152), which is present on the surface of activated T cells. B7-1/B7-2 co-stimulation of inhibitory signals occurs when B7-1/B7-2 bind CTLA-4 (Brunet et al. (1987) Nature 328: 267-270, Linsley et al. (1991) J. Exp. Med. 174: 561-569). The outcome of an immune response involves a balance between CD28 mediated T cell activation and CTLA-4 mediated T cell inhibition.
Inhibition of CD28 mediated T cell activation could inhibit undesired T cell responses occurring during autoimmunity, transplant rejection, or allergic responses. For example, inhibiting CD28 mediated T cell activation could delay graft rejection, prevent acute allograft rejection, induce donor specific tolerance, and prevent development and interrupt the progression of chronic allograft rejection, as well as prevent graft versus host disease (GVH), i.e., when transplanted T cells mount a vigorous immune response against host tissue alloantigens (Salama et al. (2001) J. Clin. Invest. 108: 943-48). Not only would inhibiting CD28 mediated T cell activation dampen the immune response through negating activation signaling through CD28, it should not impact the interaction of CD86 and CD80 to CTLA-4, thereby preserving CTLA-4 mediated inhibition of the T cell response. Thus, inhibiting CD28 mediated T cell activation could be used to prevent induction of autoimmunity and moderate the progression and/or severity of established autoimmune diseases, including models of collagen induced arthritis, autoimmune thyroiditis, autoimmune uveitis, myasthenia gravis and lupus (Saloman et al. (2001) Ann. Rev. Immunol. 19: 225-252).
What is needed is a way to inhibit CD28-mediated T cell activation, without stimulation of CD28 signaling pathways. The disclosure set forth herein meets and addresses this need.